Electronic devices, such as cellular telephones, cameras, and computers, commonly use image sensors to capture images. A typical CMOS (complementary metal-oxide-semiconductor) imager circuit includes a focal plane array of pixels, and each pixel includes a photo-sensor, such as a photogate or photodiode, for accumulating photo-generated charge in a portion of the substrate.
Digital imaging devices may produce an image with vignetting (darkening) and/or out-of-focus regions at corners and/or edges of an image, compared to the image center, for example as illustrated in FIGS. 3A and 3B. Vignetting is often an unintended and undesired effect caused by the lighting conditions of a scene, camera settings, and/or lens limitations, such as the f-number, the type of lens (e.g., wide angle lens), aperture diameter, lens defects, and/or mechanical issues. Mechanical vignetting occurs when light is partially blocked by external objects, such as thick or stacked filters, secondary lens, and/or improper lens hoods. Optical vignetting is caused by light hitting the lens aperture at a strong angle—an internal physical obstruction. This effect is often noticeable in images taken with wide angle lenses and wide open apertures. Natural vignetting, is caused by the light reaching different locations on the image sensor at different angles. This type of vignetting is most noticeable with wide angle lenses.